Time-of-flight distance measurement systems are active systems that exploit the propagation of a signal to an object and back to the sensor at a limited, but well-known speed. The kind of signal carrier typically is manifold, ultra sonic, water or electromagnetic waves. Any time-of-flight measurement system, however, works in the same principle independent on the form of the carrier used. A general set-up scheme for time-of-flight distance measurement systems is shown in FIG. 1. The distance information is calculated as:
  R  =            v      ⁢                          ⁢      T        2  
where R is the distance between the measurement system 100 and the object 10, v is the propagation speed of the signal 114 from the signal source 110 and T is the measured time needed for the signal to travel between the measurement system 100 and the object 10 forth and back and then detected by the sensor 112.
The measurement of the time-of-flight T requires the modulation of the signal 114. In optical measurement systems this is usually an intensity modulation scheme. So-called continuous wave signals such as sinusoidal or pulse-chain are widely used. Examples of these signals 114 are shown in FIG. 2.
The time-of-flight measurement is usually accomplished by correlating the detected modulation signal d(t) with a reference signal r(t). A block diagram of the receiver-wise incorporated correlator is shown in FIG. 3. The detected signal is the delayed version of the emitted signal e(t) due to its travel between the measurement system 100 and the object 10. This delay is expressed by the time-of-flight T. In addition, any losses of signal power within the measurement path are expressed by the attenuation factor k. The delay by the time-of-flight appears in the correlation curve also. By acquiring several points of the correlation curve, the delay and hence the time-of-flight is extracted.
By exploiting modern semiconductor processing technologies such as standard CMOS or CCD processes, the correlating receivers can be miniaturized down to the micrometer range. This enables the realization of large arrays of correlator elements, which are usually exploited in optical time-of-flight measurement systems for enabling the parallel measurement of the distances to some thousands of object points. Real-time creation of distance map of the surrounding becomes feasible. The correlator elements are referred to as demodulation pixels in literature.
Referring to the example of sinusoidal modulation, the correlation curve being the result of the correlation between the detected sinusoidal modulation signal and the reference signal of sinusoid with same frequency is a sinusoid again. By sampling this sinusoidal correlation curve 402 at two points S0 and S1 separated by a quarter of the modulation period 408 as shown in FIG. 4, the time of flight is extracted as:
  T  =            -                        T          m                          2          ⁢          π                      ⁢          arctan      ⁡              [                              S            ⁢                                                  ⁢            0                                S            ⁢                                                  ⁢            1                          ]            
where Tm denotes one modulation period. The practical control of the space between the two samples can easily be achieved by changing the delay of the reference signal appropriately.
In particular, sinusoidal modulation has the advantage against pulse-chain modulation that the overall hardware components of the system might be optimized to just one specific frequency, while a pulse chain based system needs to support a broader bandwidth. In any case, two major drawbacks are common for continuous wave modulation.
First, the requirement of high-frequency modulation wave for obtaining lower measurement noise is in direct contradiction with the fact that higher frequencies directly lead to reduced non-ambiguous measurement range. Here non-ambiguous measurement range means that due to the 2pi (π) wrapping nature of the continuous modulation, objects located at distances beyond half of the wavelength are seen as close objects standing at a distance less than the half wavelength. Mathematically the maximum time-of-flight, which is non-ambiguously measurable, is:Tmax=Tm 
Second, the superposition of several measurement systems' signals results in wrong measurements. Such an error cannot be compensated because it is not detectable. Thus, a system based on continuous wave modulation does not reliably function in a so-called multi-user environment.
Concerning 3D time-of-flight imaging cameras this is a major drawback in many applications where several systems need to operate simultaneously. A typical application is, for example, autonomous navigation of robots or vehicles.